Stegocephali

Stegocephali (often spelled Stegocephalia, from Greek στεγοκεφαλια, lit. "roofed head") is a clade of vertebrate animals containing all fully limbed tetrapodomorphs. It is equivalent to a broad definition of the superclass Tetrapoda: under this broad definition, the term "tetrapod" applies to any animal descended from the first vertebrate with four limbs each with five digits in the extremity (pentadactyly), rather than fins of their sarcopterygian relatives.

Stegocephali Temporal range: Late Devonian – Present, 375–0 Ma PreꞒ Ꞓ O S D C P T J K Pg N Possible Middle Devonian record
Examples of stegocephalians (clockwise from top left): Litoria phyllochroa, Acanthostega gunnari, Vulpes vulpes, and Tyto alba
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Sarcopterygii
Clade:	Tetrapodomorpha
Clade:	Elpistostegalia
Clade:	Stegocephali Cope, 1868
Subgroups
See text.

Stegocephalians include both the modern lineage of limbed vertebrates (the crown group tetrapods, including modern amphibians, mammals, reptiles and birds) as well as a portion of the stem group, the earliest limbed tetrapodomorphs such as Ichthyostega and Acanthostega, which evolved in the Devonian period long before the origin of the crown group. Many paleontologists prefer a stricter definition of Tetrapoda which applies solely to the crown group, excluding earlier types of limbed tetrapodomorphs. Stegocephali was re-established to replace the broad definition of Tetrapoda, resolving the usage of two conflicting definitions in discussions of tetrapod evolution.

Stegocephali was coined in 1868 by the American paleontologist Edward Drinker Cope, who used it as a general category of prehistoric amphibians. This name was in reference to the skull form of many early tetrapods, with a low, solid shape combining numerous strongly-textured dermal bones. In its original usage, the term quickly became obsolete. In 1998, Canadian paleontologist Michel Laurin repopularized the term and provided a formal phylogenetic definition as a monophyletic clade containing both crown-group and stem-group tetrapods. Laurin's Stegocephali is roughly defined as including all vertebrates closer to modern tetrapods than to Panderichthys. This definition was intended to include taxa with digits rather than fins, except where secondarily lost. Another definition, published in Phylonyms, defines the group as including all taxa closer to Eryops than to Tiktaalik, Panderichthys, or Eusthenopteron. The discovery of the Zachelmie trackways in 2010 suggests that stegocephalians possibly emerged 395 Ma or earlier.

 

Originally, the term was used as a systematic unit at the rank of order. The term rose to prominence in American and British science in the 19th century, though the largely equivalent term Labyrinthodontia had been coined 18 years earlier by Hermann Burmeister in reference to the tooth structure. The terms were used interchangeably during the early 20th century, usually divided into three orders. Cope originally spelled the term as "Stegocephali", though A.S. Woodward introduced a popular alternative spelling, "Stegocephalia", in 1898. In their original usage, Stegocephali (and the Labyrinthodontia) are paraphyletic, the name is now used in an informal way to denote the early non-piscine vertebrates, excluding amniotes (the first reptiles and their descendants) and modern lissamphibians.

Paleontologist Michel Laurin took up the older term and defined stegocephalians phylogenetically as all vertebrates more closely related to Temnospondyli than to Panderichthys (the closest relative of tetrapods known to have retained paired fins, see below). Therefore, Stegocephali includes all vertebrate groups that have toes rather than fins, and a few (Elginerpeton, Metaxygnathus, Ventastega and possibly Hynerpeton) that may have retained paired fins. Contrary to the old usage of this term, the Stegocephali refers to a clade in this scheme. This concept of the clade Stegocephali was chosen to substitute for the name Tetrapoda by those who sought to restrict Tetrapoda to the crown group. As such, it encompasses all presently living land vertebrates as well as their early amphibious ancestors.

Below is an evolutionary tree of stegocephalians, as recovered from an analysis by Swartz in 2012.

As recovered by Clack et al. 2016:

• †Elginerpetontidae
• †Aistopoda
• †Whatcheeriidae
• †Colosteidae
• †Adelospondyli
• †Baphetoidea
• †Embolomeri (a.k.a. Anthracosauria sensu stricto)
• crown-Tetrapoda
  • Batrachomorpha (stem-Lissamphibia, likely equivalent to Temnospondyli)
  • Reptiliomorpha (stem-Amniota, a.k.a. Anthracosauria sensu lato)
• Stegocephali incertae sedis
  • †Acanthostega
  • †Antlerpeton
  • †Aytonerpeton
  • †Brittagnathus
  • †Crassigyrinus
  • †Diploradus
  • †Densignathus
  • †Doragnathus
  • †Elpistostege?
  • †Hynerpeton
  • †Ichthyerpeton
  • †Ichthyostega
  • †Jakubsonia
  • †Koilops
  • †Livoniana?
  • †Mesanerpeton
  • †Metaxygnathus
  • †Occidens
  • †Ossirarus
  • †Parmastega
  • †Perittodus
  • †Sigournea
  • †Sinostega
  • †Tantallognathus
  • †Tiktaalik?
  • †Tulerpeton
  • †Tutusius
  • †Umzantsia
  • †Ventastega
  • †Ymeria